1. Field of the Invention
This invention relates to rear window defoggers which are installed on the inside surface of a motor vehicle's rear window for increasing the temperature of the rear window in order to clear the same of fog or frost and, more particularly, to such defoggers comprised of a plurality of narrow electrically conducting lines and an improved method and composition for repairing such lines in which cracks or other discontinuities render the lines non-conductive.
2. The Prior Art
Rear Window defrosters have become quite popular in motor vehicles used in colder climates and fall into two distinct classes, namely the factory installed version and systems which are sold in kit form to be installed on the rear window inside a car. The factory installed systems will last for many years, but they have the disadvantage that the very fine lines of conductive material tend to be quite fragile and can be easily damaged as when cleaning the window with an abrasive substance or when some sharp object is unintentionally allowed to move against the inside surface of the window.
The other variety of rear window defrosters is a defroster kit of the transfer type wherein a very thin foil of conductive material is transferred from a clear transparent support that eventually will be removed leaving only a pattern of relatively wide silver looking bands on the inside surface of the window, and some attachment to make the connection to the motor vehicle's electrical system. Passage of a current through the lines of the system generates sufficient heat to dissipate fogging or frost that would otherwise accumulate against the inside surface of the rear window. The heating elements of this system are extremely fragile, and the individual heating lines are also easily broken or interrupted resulting in an unsatisfactory defogging and defrosting of the rear window.
Compositions generally used to repair interrupted electrical heating lines in rear window defrosters are conductive glues which are used to bridge the discontinuity and re-establish conductivity thereacross. Such glues are generally comprised of a mixture about 50 to about 85 percent by weight of a conductive metallic pigment or flake such as silver or copper and about 15 to about 50 percent by weight of an adhesive binder such as an epoxy resin or solvent borne thermoplastics. Although these glues are effective for repairing small breaks in the conductive lines of the defroster, in the event the non-conductive area is a break or gap of appreciable length, the glue does not possess the requisite electrical conductivity to provide sufficient electrical energy to produce a suitable heating output in the repaired conductive line to maintain the window free from condensation or frost.